One thing that causes me problems is when I have a crowded diagram with many arrows pointing to a same box - it would be nice if such arrows could be merged into a single arrow representing them all. This is also useful when a diagram represents a process with several subprocesses and the results are merged to provide one single output.

(e.g., arrows "red", "blue" and "green" merge into a "color" arrow).

Similarly, the opposite would be also nice, i.e., a "color" arrow splitting into different arrows representing individual "colors".

This is particularly interesting when depicting data sets from different sources that for example leave the current diagram and are then used in different diagrams - it is easier to depict one single arrow leaving/entering a diagram than a dozen...

This could IMHO be implemented as an arrow linking to many arrows where the "green", "blue" and "red" arrow heads would for example link to the base of the "color" arrow. That is, the "color" arrow would be a different arrow that would combine the arrows of different "colors".

In addition, it would be nice if there was the possibility to create a same arrow with multiple bases and/or heads. This would for example allow having one "same" arrow going into different boxes. The difference with the previous suggestion is that one could identify the arrow as being the same, instead of a set of different arrows. Perhaps this could be achieved by means of an arrow collection?